Astronomers: gamma ray death from above 'unlikely'

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A gamma-ray burst (GRB) in our galactic neighbourhood could decimate life, destroy the ozone layer and trigger drastic climate change. A new study of Hubble data, published in Nature, has found such a cosmic deathray scenario is less likely than previous doomsday predictions.

The radiation the international team investigated comes with some of the biggest star explosions in the universe – long GRBs, which occur when a core-collapse supernova triggers an even more violent explosion.

Supernovae occur all over the universe and throughout galaxies. The researchers were testing the assumption that the galaxies with most supernovae would suffer the most GRBs. Instead, they found that long GRBs tend to originate from the runts of the cosmic litter – small, faint, irregularly shaped galaxies (pictured below; crosshairs indicate locations of GRBs).

Our own Milky Way is a regular spiral, so GRB armageddon from a nearby star death is unlikely, they reckon. Out of 42 GRBs measured, the astronomers found just one in a Milky Way-like spiral.

Lead author Andrew Fruchter of the Space Telescope Science Institute said: “[GRBs] occurrence in small irregulars implies that only stars that lack heavy chemical elements tend to produce long-duration GRBs.”

Heavy elements are themselves produced in supernovae and have built up in the universe over time. The implication is that long GRBs are getting less common in the universe generally.

They also found that unlike standard supernovae, long GRBs are concentrated at their host galaxy's brightest region. Study co-author Andrew Levan explained: "The discovery that long-duration GRBs lie in the brightest regions of their host galaxies suggests that they come from the most massive stars – perhaps 20 or more times as massive as our Sun."

The team concludes that GRBs are "relatively rare" in the Milky Way. ®